Plasma processing has long been employed to process substrates (e.g., wafer or flat panels or other substrates) to create electronic devices (e.g., integrated circuits or flat panel displays). In plasma processing, a substrate is disposed in a plasma processing chamber, which employs one or more electrodes to excite a source gas (which may be an etchant source gas or a deposition source gas) into a plasma for processing the substrate. The electrodes may be excited by an RF signal, which is furnished by a RF generator, for example.
In some plasma processing systems, multiple RF signals, some of which may have the same or different RF frequencies, may be provided to one or more electrodes to generate plasma. In a capacitively-coupled plasma processing system, for example, one or more RF signals may be provided to the top electrode, the bottom electrode, or both in order to generate the desired plasma.
In some applications, the RF signals may be pulsed. For any given RF signal, RF pulsing involves turning the RF signal on and off, typically within the same RF signal period but may span multiple RF signal periods. Furthermore, the RF pulsing may be synchronized among signals. For example, if two signals RF1 and RF2 are synchronized, there is an active pulse of signal RF1 for every active pulse of signal RF2. The pulses of the two RF signals may be in phase, or the leading edge of one RF pulse may lag behind the leading edge of the other RF pulse, or the trailing edge of one RF pulse may lag behind the trailing edge of the other RF pulse, or the RF pulses may be Out of phase.
In the prior art, pulsing synchronization of multiple RF signals typically involves a communication network to facilitate control communication anions the various RF generators. To facilitate discussion, FIG. 1 is as high level drawing of a generic prior art implementation of a typical pulsed RF plasma processing system 102. Pulsed RF plasma processing system 102 includes two RF generators 104 and 106. In the example of FIG. 1, RF generator 104 represents a 2 MHz generator while RF generator 106 represents a 60 MHz generator.
A host computer 110 implements tool control and receives a feedback signal 112 from an impedance matching network 114 to provide (via a digital or analog communications interface 116) power set point data to RF generator 104 and RF generator 106 via paths 118 and 120 respectively. The feedback signal 112 pertains to the impedance mismatch between the source and the load, and is employed to control either the delivered power or the forward power levels of RF generators 104 and 106 to maximize power delivery and minimize the reflected power.
Host computer 110 also provides Pulse_Enable signal 160 to a pulse synchronizer and controller 130. Responsive to the Pulse_Enable signal 160, the pulse synchronizer and controller 130 provides the synchronized control signals 170 and 172 to RF generator 104 and RF generator 106 (via External Synchronization Interfaces 140 and 142) to instruct RF generators 104 and 106 to pulse its RF signals using power controllers 150 and 152 respectively to produce pulsed RF signals 162 and 164. The pulsed RF signals 162 and 164 are then delivered to the load in plasma chamber 161 via impedance matching network 114.
Although the pulsed RF synchronization scheme of FIG. 1 can provide the synchronized pulsing function for the RF generators, there are drawbacks. For example, synchronizing the pulsing function of the various RF generators in FIG. 1 requires the use of a network to communicate among host computer 110, pulse synchronizer/controller 130, and external synchronization interfaces 140 and 142 in RF generators 104 and 106. Further, synchronizing the pulsing function of the various RF generators in FIG. 1 requires the implementation of the external synchronization interfaces such as 140 and 142) in the various generators. Implementing these external synchronization interfaces adds an extra layer of complexity to RF generator designs, and render existing RF generators incapable of being used for RF synchronized pulsing,
In view of the foregoing, there are desired improved techniques and systems for implementing synchronized RF pulsing in a plasma processing system.